A method of producing a semiconductor laser is provided which comprises a first time crystal growth process for producing a crystal growth layer (1, 2 3) including at least a semiconductor layer (2) including aluminum on a substrate (1), a process for producing a stripe groove (10) by conducting a selective etching to the crystal growth layer to expose a portion of the semiconductor layer (2) including aluminum, and a second time crystal growth process for producing a semiconductor layer (4) including no aluminum on the entire wafer surface including the exposed surface of the semiconductor layer (2) and further producing a semiconductor layer (5) including aluminum on the semiconductor layer (4) including no aluminum. The production of a high resistance layer (9) having bad crystallinity at the second time crystal growth is thus prevented.
The present invention relates to a method of producing a semiconductor laser, and more particularly to that enables obtaining a high reproducibility and high reliability semiconductor laser.
A prior art SBA laser is reported by Mihashi et al in a preprint of Associated Spring Meeting of Applied Physics Society related societies of 1985 30a-ZB-4.
Figure 2 shows a cross-sectional view of an SBA (Self aligned laser with Bent Active layer) laser that is produced by utilizing MO-CVD method.
In this figure, the reference numeral 1 designates a p type GaAs substrate, the reference numeral 2 designates a buffer layer comprising p type Al0.43Ga0.57As, the reference numeral 3 designates a current blocking layer comprising n type GaAs, the reference numeral 5 designates a first cladding layer comprising p type Al0.43Ga0.57As, the reference numeral 6 designates an active layer comprising undoped Al0.07Ga0.93As, the reference numberal 7 designates a second cladding layer comprising n type Al0.43Ga0.57As, the reference numeral 8 designates a contact layer comprising n type GaAs, the reference numeral 9 designates an absorbed oxygen film, the reference numeral 10 designates a stripe groove which is produced in a reverse trapezoid cross section at the current blocking layer 3. The reference numeral 11 designates an active region, the reference numerals 12 and 13 designate an n electrode and a p electrode fixed to the contact layer 8 and the substrate 1, respectively.
The method of producing this SBA laser will be described briefly.
At first, a buffer layer 2 and a current blocking layer 3 are grown on the substrate 1 by MO-CVD method in a first crystal growth. After the growth, a stripe groove 10 having a reverse trapezoid cross section is produced at the current blocking layer 3 using an etchant for etching only GaAs selectively. As a result of this selective etching, the buffer layer 2 comprising p type Al0.43Ga0.57As is exposed at the bottom of the strip groove. Next, in the second crystal growth using MO-CVD method a first cladding layer 5, an active layer 6, a second cladding layer 7, and a contact layer 8 are grown successively on the wafer having the strip groove 10. After the second crystal growth, an n type electrode 12 and a p type electrode 13 are produced on the contact layer 8 and at the surface of the substrate 1, respectively, by a method of such as vapor plating or spattering, thereby completing a SBA laser.
In this SBA laser, a current flows through the stripe groove 10 produced at the current blocking layer 3, and the portion parallel with the substrate 1 positioned above the stripe groove 10 of the active layer 6 becomes an active region 11. Furthermore, by using a MO-CVD method, the active layer 6 can be bent in a confirguration close to that of the stripe groove 10. In this bent portion, there arises a refractive index difference in the transverse direction, and thus a light confinement in the transverse direction is conducted effectively, thereby resulting in a low threshold current and a high efficiency.
In this prior art method of producing a SBA laser, a buffer layer 2 comprising p type Al0.43Ga0.57As is exposed at the bottom of the stripe groove 10 at the etching for producing the stripe groove 10. Meanwhile, as Al has a nature to be easily combined with oxygen, a thin oxide layer is produced at the surface of the buffer layer 2 by an etching or a water washing process. Further, it is exposed to the air until a second crystal growth is conducted, and the oxidation proceeds with the passage of time. Generally such an oxide film is not a solid one, and almost all the portion thereof is diversified and forfeited in the temperature rising process at the growth of MO-CVD method, but occasionally quite thin oxide layer remains locationally, or an absorbed oxygen film 9 remains that is resulted from a high concentration of oxygen combined with aluminum, even if an oxide layer is not resulted.
Oxygen is taken into the crystal when a first cladding layer 5 comprising p type Al0.43Ga0.57As is directly grown on the buffer layer 2, thereby producing a high resistance layer including oxygen which deteriorates the crystalinity. Such a reduction in crystalinity and production of a high resistance layer worsen the reproducibility of the laser characteristics as well as invite characteristics deterioration such as rising ups of threshold current and operational current. Furthermore, the temperature rising at operation is increased, thereby inviting deterioration of an element, affecting an unfavourable influence on the reliability.
It is an object of the present invention to provide a method of producing a semiconductor laser capable of preventing deterioration in the characteristics and reduction in the reliability without oxidating the bottom of the stripe groove at the etching for producing a stripe groove.
Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
According to the present invention, a stripe groove is produced by conducting a selective etching to the semiconductor layer which is obtained by a first crystal growth, a semiconductor layer including Al is thus exposed, and a semiconductor layer including Al is produced on the semiconductor layer including Al via semiconductor layer including no Al by a second crystal growth process. Then, the production of a high resistance layer having bad crystalinity in the second crystal growth is prevented, and a semiconductor laser superior in reproducibility and reliability is obtained.
An embodiment of the present invention will be described with reference to figure 1.
Figure 1 shows a semiconductor laser produced by a method as an embodiment of the present invention. In Figure 1, the same reference numerals designate the same elements as those shown in Figure 2. The reference numeral 4 designates a buffer layer comprising p type GaAs as a semiconductor layer including no Al.
Next, the production method will be described.
At first, in the first crystal growth a buffer layer 2 and a current blocking layer 3 are successively grown on the substrate 1. After the growth, a stripe groove 10 is produced by photolithography technique and an etching in a similar manner to that in the prior art method. Since the buffer layer 2 is exposed by this etching, an absorbed oxygen film 9 combined with Al is produced at the surface thereof. A buffer layer 4, a first cladding layer 5, an active layer 6, a second cladding layer 7, and a contact layer 8 are successively grown thereon in the second crystal growth. After the growth of these layers, an n type electrode 12 and a p type electrode 13 are produced on the contact layer 8 and at the surface of the substrate 1, respectively, by vapor plating or spattering. The thickness of the buffer layer 4 is desirable to be larger than 0.01 µm and less than the depth of the stripe groove 10, that is, the thickness of the current blocking layer 3.
Also in this invention method, in a similar manner to that in the prior art method, high concentration oxygen is absorbed to the surface of the buffer layer 2 or an oxide film is produced thereon by etching or water washing at the production of the stripe groove 10. Furthermore, the buffer layer 2 at the bottom of the groove is also exposed to the outside air after the stripe groove 10 is produced, thereby making oxygen absorbed thereto, and the absorbed oxygen film 9 results to remain at the start of the second crystal growth.
In the present invention, however, since a buffer layer 4 comprising p type GaAs is grown on the buffer layer 2 before growing a first cladding layer 5 comprising p type Al0.43Ga0.57As, the production of a high resistance layer at the surface of the crystal growth is suppressed. This is because the oxygen absorbed to the buffer layer 2 is less taken into the crystal due to that the buffer layer 4 does not include Al that is easily oxidated. This makes it possible to obtain a semiconductor laser having a good element characteristics.
Al is described in the above-illustrated embodiment in which an SBA laser is employed the present invention is effective when an AlGaAs layer is regrown by the second crystal growth on the AlGaAs layer produced by the first crystal growth. This regrowing of layers via GaAs buffer layer makes it possible to enhance the element characteristics easily.
As is evident from the foregoing description, according to the present invention a stripe groove is produced by conducting a selective etching to the semiconductor layer which is obtained by a first crystal growth, a semiconductor layer including Al is thus exposed, and a semiconductor layer including Al is produced on the semiconductor layer including Al via semiconductor layer including no Al by a second crystal growth process. Therefore, the production of a high resistance layer having bad crystalinity in the second crystal growth is prevented, and thus a semiconductor laser superior in reproducibility and reliability is obtained.